Bioremediation is a process by which living organisms prevent or reduce the negative effects of toxins and pollutants in the environment. It has emerged as an effective yet relatively inexpensive way to clean up toxic waste sites and polluted environments. Bioremediation can be applied to a wide range of toxins in various environments and has consequently become the focus of much attention. Bioremediation is desirable due to its relatively low cost, its effectiveness, and because it utilizes living organisms that are native to the site of remediation. As a non-limiting example of a pollutant suitable for bioremediation, Chromium (VI) is extremely toxic, carcinogenic, and mutagenic, but commonly enters the soil and groundwater as an unwanted byproduct of industrial processes. Therefore, it is important to develop techniques to remediate contaminated areas containing this toxin.
Many naturally occurring bacteria have been found to detoxify the environment by reducing toxins such as Uranium (VI) to Uranium (IV), Arsenic (VI) to Arsenic (III), Lead (II) to Lead (0), and Chromium (VI) to Chromium (III). Reducing these contaminants makes them less soluble and less, likely to contaminate water supplies.
While many bacteria such as Escherichia coli (E. coli) have a natural ability to detoxify contaminated surroundings and create habitable environments, current research aims to improve the detoxification capabilities of these bacteria by increasing their viability in toxic environments. By increasing viability, their detoxification power becomes more effective, and thus reasonably low quantities of these microorganisms could be successfully applied to large-scale clean ups. Therefore, it is important to explore the roles and applications of various molecules and enzymes that can be manipulated in these bacteria to enhance bioremediation. A particularly interesting molecule that has tremendous potential in this field is trehalose, a sugar that has been found to improve survival of toxin-exposed organisms.
Trehalose, which is found in plants, animals, and some microorganisms, is a compatible solute composed of two α, α, 1, 1 linked glucose molecules. Compatible solutes are small molecules that can build up in high concentrations without disrupting normal biological processes. This accumulation protects cells against damage from desiccation, high salt concentrations, oxidative stress, or extreme thermal conditions. Trehalose is a unique molecule in that it appears to protect against each of these types of stresses. In bacteria, trehalose accumulates as a biological adaptation to resist stress. Trehalose is therefore particularly attractive for the field of bioremediation because of its potential to facilitate improved bioremediation through protection of bacteria from different types of pollutants and toxins causing stress.
A number of trehalose biosynthesis pathways are naturally found in different microorganisms. Many bacteria such as E. coli contain only the OtsA/OtsB pathway which generates trehalose from glucose-6-phosphate and UDP-glucose (FIG. 1). The OtsA enzyme is responsible for production of trehalose-6-phosphate synthase while OtsB encodes trehalose-6-phosphate phosphatase. Other organisms such as Mycobacterium smegmatis (M. smegmatis) contain multiple trehalose biosynthesis genes such as TreY/TreZ and TreS in addition to the OtsA/OtsB pathway. The TreY/TreZ pathway generates trehalose using maltoheptose, with the TreY gene encoding maltooligosaccharyltrehalose synthase and the TreZ encoding maltooligosaccharyltrehalose trehalohydrolase (FIG. 1). The TreS pathway uses trehalose synthase to generate trehalose from maltose (FIG. 1). The presence of multiple pathways indicates the integral role of trehalose in the survival of these bacteria.
Researchers have devised numerous schemes to enhance bioremediation by increasing the rate at which it occurs. However, the quest remains to achieve more effective bioremediation through development of methods that significantly improve the detoxification capabilities of bacteria.
Previous studies have demonstrated that E. coli is capable of reducing the toxin Chromium (VI) to its much less toxic form Chromium (III). Chromium is commonly used in the production of nuclear weapons and for many industrial purposes, and is thus a prevalent environmental contaminant. Chromium (VI) is highly soluble and extremely toxic, carcinogenic, and mutagenic. To minimize the detrimental effects of this contaminant it can be reduced by microorganisms to its trivalent form, which is known to be less soluble and 1000-fold less mutagenic. However, reducing Chromium from its hexavalent to trivalent state generates reactive oxygen species (hydroxyl radicals) that are damaging to bacteria. Therefore, it is important to defend these bacteria from the associated oxidative stress in order to maintain or increase the rate of bioremediation. Given the protective nature of trehalose, increased trehalose production should provide bacteria with an added defense against the oxidative stress generated from Chromium (VI) reduction.